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Precipitation titration - equivalence point calculation

Precipitation

Argentometry

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Analytical Chemistry: Theoretical and Metrological Fundamentals Amazon

Analytical Chemistry: Theoretical and Metrological Fundamentals

by K.Danzer

Precipitation titration » Equivalence point calculation

At equivalence point we have just a saturated solution of insoluble salt, so calculation of concentration of the determined ion is identical to the solubility calculations. Just for fun, we can derive seriously looking formula that will describe the concentration.

Let's assume we precipitate salt of a general formula MekXl, with known Ksp:

precipitation-titration-equivalence-point-calculation - solubility product, eq. 11

Looking at the formula we can see that

precipitation-titration-equivalence-point-calculation, eq. 22

so

precipitation-titration-equivalence-point-calculation, eq. 33

and

precipitation-titration-equivalence-point-calculation, eq. 44

However, in practice you will probably find it easier to solve the problem manually each time, than to look for formula or trying to remember it. That's what we will do in the following example.

Note, that in the real world it is quite often necessary to account for numerous side reactions - especially for protonation and hydrolysis of both metal cation and ligand.

Example: calculate pZn at the equivalence point of zinc titration with ferrocyanide, assuming pKsp = 16.8.

We don't need any other information about volumes and concentrations! Let's start with dissolution reaction and Ksp:

Zn2Fe(CN)6 → Zn2+ + Fe(CN)64-

precipitation-titration-equivalence-point-calculation, eq. 55

obviously

precipitation-titration-equivalence-point-calculation, eq. 66

so

precipitation-titration-equivalence-point-calculation, eq. 77

and

precipitation-titration-equivalence-point-calculation, eq. 88

and finally

precipitation-titration-equivalence-point-calculation, eq. 99

Let's check, if our general formula gives the same result. From the Zn2Fe(CN)6 formula k=2 and l=1, so

precipitation-titration-equivalence-point-calculation, eq. 1010

That's the same.


Page was last modified on February 24 2009, 11:25:02.

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